A method whereby seawater/salt water or the like is introduced into and subsequently compressed in a pressure vessel such as the one disclosed in Patent Document 1, for example, loaded with a hollow fiber membrane or spiral membrane, and separated into fresh water and concentrated seawater is commonly used in reverse osmosis seawater desalination plant equipment.
Since seawater is introduced into the pressure vessel and concentrated seawater is conducted out of the pressure vessel, extremely high corrosion resistance (rust resistance) is needed in the pressure vessel in contact with the seawater and concentrated seawater, the closure lid for closing the end parts of the pressure vessel, and the ducts connected to the pressure vessel.
In a reverse osmosis method for converting seawater into fresh water, the inside of the pressure vessel is continuously subjected to a pressure of 800 to 120 psi (5.5 to 8.3 MPa), which is 5.5 to 8.3 times the 1 MPa pressure of compressed air generated by a common compressor.
The pressure vessel for converting seawater into fresh water must also pass the Section X qualification test specified by the ASME (American Society of Mechanical Engineers). Specifically, according to the Section X qualification test, the pressure applied to cause the pressure vessel to break after pressurization to the operating pressure of 800 to 1200 psi (5.5 to 8.3 MPa) and depressurization are repeated 10,000 times must be six or more times the pressure of 800 to 1200 psi (5.5 to 8.3 MPa). Specifically, the pressure vessel for converting seawater into fresh water must have a pressure resistance of 4800 psi (33 MPa) or higher.
The ducts connected to the pressure vessel are therefore constructed using stainless steel, particularly pipes made of super stainless steel such as 254SMO, YUS270, SAF2507, or Super Duplex, having excellent corrosion resistance.
Specifically, reverse osmosis requires an RO module composed of a vessel (pressure vessel) and an RO element, and a duct for connecting these components. The RO module used to convert seawater into fresh water is usually subjected to a pressure equivalent to 6.895 MPa (1000 psi) or higher, and since the ducts used in the RO module are also subjected to a pressure equivalent to 6.895 MPa (1000 psi) or higher, the design must take pressure resistance into consideration.
For example, a seawater desalination device such as shown in FIG. 1 is formed in which a plurality of RO modules 42 is installed in a frame 41, and the ends of the RO modules 42 are provided with manifolds 43 composed of seawater introduction tubes 44 for introducing seawater, concentrated seawater outlet tubes 45 for discharging the concentrated seawater after the seawater has been separated into concentrated seawater and fresh water, and other components. The seawater introduction tubes 44, concentrated seawater outlet tubes 45, and other components are also constructed using pipes made of stainless steel such as 254SMO, YUS270, SAF2507, and Super Duplex, for example, as with the pressure vessel. The reference numeral 46 in FIG. 1 refers to fresh water outlet tubes for discharging fresh water.